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Shuttle Atlantis counting down to Sunday launch tryBY WILLIAM HARWOODSTORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSIONPosted: December 8, 2007

NASA's Mission Management Team today cleared the shuttle Atlantis for a second launch attempt Sunday, but agreed that any additional problems with suspect low-level hydrogen fuel sensors in the ship's external tank will trigger another delay.

In addition to requiring that all four engine cutoff - ECO - sensors in the shuttle's hydrogen tank work flawlessly, the MMT also said new instrumentation that monitors the status of the sensors must be in perfect working order as well.

For the first time, data from that instrumentation will be used during ascent to give flight controllers insight into the health of the sensors and the shuttle's launch window will be reduced from five minutes to just one minute to ensure Atlantis takes off with enough fuel to protect against worst-case failures that otherwise might drain the tank faster than expected.

"Tomorrow, we've decided to go tank and if everything works perfectly, as we would expect from our past history, we'll go fly," said shuttle Program Manager Wayne Hale. "And if we have any other anomalies, or a repeat of an anomaly, then we think it is worthwhile to stand down and troubleshoot. The nature of that troubleshooting and how long it would take is a little bit open ended. We'll wait to see what happens before we decide that. But suffice it to say we would not be launching tomorrow and it probably would reduce our chances of launching in the December launch window substantially."

But if the hardware cooperates, "we're going to give it a shot tomorrow," he said.

Launch is targeted for 3:21:00 p.m., roughly the moment Earth's rotation carries the launch pad into the plane of the space station's orbit. Forecasters predict an 80 percent chance of good weather Sunday and 70 percent on Monday.

The shuttle has enough fuel to launch five minutes to either side of the "in-plane" time and still catch up with the space station. But NASA typically targets the middle of the 10-minute window, giving a crew just five minutes or so to get off the ground.

For Atlantis' launch, the window has been shortened to just one minute beyond the 3:21 p.m. in-plane time. Launching later in the window would require more fuel to catch up with the space station and NASA managers want to ensure Atlantis has ample reserves to avoid any chance of an engine shutdown due to propulsion problems and any possible trouble with the low-level fuel sensors.

Three of four engine cutoff sensors in the hydrogen section of the shuttle's external tank failed to respond properly during tests to verify their health shortly after fueling began for an initial launch attempt last Thursday. The sensors later returned to normal operation and NASA managers tentatively decided late Friday to set up for another launch attempt Sunday.

The ECO sensors are part of a backup system intended to detect low fuel levels in time to prevent the shuttle's main engines from draining the tank after some other problem that might cause the powerplants to use up hydrogen faster than expected. Running out of fuel during engine operation likely would trigger a catastrophic failure.

In the wake of earlier problems with the cutoff sensors, NASA implemented a flight rule change that would permit a countdown to proceed if three of four sensors were operating normally and certain other conditions were met.

But engineers do not yet know what caused two sensors to "fail wet" during fueling Thursday or why a third sensor did the same during de-tanking operations. Because the system is suspect, the flight crew office proposed proceeding with launch Sunday if, and only if, all four sensors are operating normally.

The health of the sensors will be monitored throughout the countdown, using computer commands to simulate wet and dry conditions. The first such 'sim test" will occur early in the fueling process with additional checks planned at various points in the countdown. Voltage readings will be monitored all way to the T-minus 31-second mark when the shuttle's flight computers take over the countdown.

Using the new instrumentation, flight controllers will be able to determine whether any sensors fail after launch, allowing the crew to take action to preclude any potentially catastrophic downstream failures. Those options include aborting to a lower-than-planned orbit, dumping maneuvering fuel overboard to lighten the ship or diverting to an emergency landing in Spain or France.

All of those worst-case scenarios, Hale stressed, would require multiple failures in unrelated systems. As such, program managers and engineers decided it was safe to proceed with launch.

There is some reason to believe the sensors will work normally the second time around. Similar problems have gone away during past launch campaigns after initial exposure to minus 423-degree hydrogen fuel. Whether all four sensors will cooperate Sunday remains to be seen.

But Launch Director Doug Lyons said anything less than normal behavior from the sensors and the new instrumentation that monitors their performance will stop the countdown and result in another launch delay. Whether NASA would make any additional launch tries before the current window closes Dec. 13 is not yet known.

Today's decision to proceed with another launch try, even with more restrictive launch rules and requirements, was opposed by some engineers who argued for standing down and conducting additional tests to determine what might be causing the problem.